Epidemiology of reproductive performance of dairy cows
in commercial herds in Australia
John Morton BVSc (Hons) Ph D MACVS
Senior Lecturer in Veterinary Epidemiology and Biometry
School of Veterinary Science, University of Queensland, Brisbane, Australia
Reproductive performance of lactating dairy cows was studied using a prospective observational study of 29,462 cows in 168 commercial herds in 9 regions in Australia. There was substantial variation in all reproductive performance measures between herds, suggesting that important increases in reproductive performance would be possible in many commercial Australian herds if causal factors were identified and modified.
Multivariable multilevel logistic models were used to study potential determinants for two measures of overall reproductive performance - pregnancy by week 6 after start of the mating period (mating start date or MSD) and non-pregnancy by week 21 after MSD, and for two components of overall reproductive performance - submission by week 3 after MSD and conception to first service.
The odds of pregnancy by week 6 and/or week 21 after MSD were substantially higher amongst cows with longer calving to MSD intervals and higher milk protein concentrations. Reproductive performance was reduced amongst both young and old cows, cows with poor previous reproductive performance, cows with particular clinical diseases at calving or in early lactation and cows observed in oestrus in the first 3 weeks after MSD but not inseminated. Crossbred (50% Holstein-Friesian/ 50% Jersey) cows had modestly higher reproductive performance than Holstein-Friesian cows. Cows whose parturition had been induced had similar reproductive performance to non-induced cows and only small reductions in reproductive performance were observed amongst both low and high fat-yielding cows. Cows sired by high genetic merit bulls had similar phenotypic reproductive performance to other cows.
Herd ovulation detection probability was strongly associated with odds of pregnancy by week 6 after MSD but low herd positive predictive values for oestrus diagnoses (less than 0.95) were associated with only modest reductions in odds of pregnancy by week 6 after MSD. Lower-producing herds had poorer reproductive performance and the odds of non-pregnancy were increased with short mating periods.
Certainty of oestrus diagnosis, artificial insemination (AI) technician type (professional or farm technician) and service bull determined the odds of conception to first service and so may indirectly determine chance of pregnancy by week 6 and/or week 21 after MSD. Odds of conception were similar for short and medium intervals between observation of oestrus and insemination.
Further research is required to identify mechanisms for reduced reproductive performance amongst cows with lower milk protein concentration, relative to cows with higher concentrations, and amongst young and old cows.
To identify exposures of most importance in the study population, population attributable fractions and risks were estimated for factors associated with pregnancy by week 6 after MSD. For lactations in seasonal calving herds, short calving to MSD intervals and low milk protein concentration had highest population attributable fractions and risks, while in year-round calving herds, low milk protein concentration, long intercalving intervals prior to the study and low ovulation detection probability had highest population attributable fractions and risks. Strategies that substantially reduce the prevalence of these exposures and/or ameliorate their effects could increase reproductive performance of the study population by important amounts.